1. Field of the Invention
The invention relates generally to the global positioning system (GPS) and specifically to methods and apparatus for taking the measurement of differential phase of two or more signals from two or more antennas receiving the same GPS satellite carrier so that the attitude may be determined for a platform or vehicle on which the GPS antennas are mounted.
2. Description of the Prior Art
The United States has placed in orbit a constellation of satellites, as part of a global positioning system (GPS), that can be used by civilians and the military alike to get automated and highly-accurate earth position coordinates on easy to read digital displays. Determining where you are has been a particular problem for seafarers for thousands of years. Now, GPS enables small sailboat owners and even individual combat soldiers to get their positions to within a few meters, using handheld portable equipment.
GPS-based attitude determination offers significant cost savings in applications where inertial guidance has traditionally been the standard approach. Attitude is measured by differential measurements of GPS carrier phase between two or more antennas. Performance may be characterized in terms of accuracy and bandwidth, both being dependent on applications specific parameters, such as the antenna spacing, multipath signal interference and the GPS signal carrier-to-noise ratio.
While there are a number of methods in the prior art for measuring the relative phase angle of a carrier received from a pair of antennas, most consist of two nearly identical hardware channels, one for each antenna. The relative phase is then derived from the respective channels. For background, the reader is referred to U.S. Pat. Nos. 4,807,256, issued Feb. 21, 1989 to Holmes, et al.; 4,485,383, issued Nov. 27, 1984, to Maher; and 4,468,793l, issued Aug. 28, 1984, to Johnson, et al. An informative article on this subject matter was also published by C. Johnson, et al., "Applications of a Multiplexed GPS User Set," an article reprinted from the Institute of Navigation (Navigation June 1981), Vol.II, 1984 ISBN: 0-936406-01-1, pp.61-77.
Multiple channel systems have inherent biases and are unavoidably duplicative and therefore expensive to manufacture and maintain. A less expensive alternative is therefore needed to make practical applications of GPS attitude determination possible.